Boron nitride commonly is used as a release agent or stop-off in many forming operations. Sometimes boron nitride is used in conjunction with other release coatings to ensure that the worksheets being formed can be separated from the dies. For example, Agrawal et al. U.S. Pat. No. 4,269,053 (issued May 26, 1981) describes using release coatings of two different coefficients of friction in SPF operations. Agrawal teaches using boron nitride and yttria as the two preferred release agents so that the workpieces of"reactive" metals like titanium or zirconium do not react with the metal of the forming member (i.e., die). Agrawal discusses that single release coatings of yttria, boron nitride, graphite, or alumina in an organic binder applied by spraying or silk screening are often used in SPF operations. Yttria alone results in a non-uniform thickness in the cup portion of the formed product. Boron nitride alone produces local necking in the radius area. Agrawal overcomes these problems by applying yttria to the areas of the preform (i.e., the worksheet(s)) which undergo little or no expansion while boron nitride is applied in the areas which undergo extensive expansion. Typically, Agrawal applies the release coatings directly to the die, but may apply it to the preform to reduce disruption of the loading of the SPF presses. Applying the coatings to the preform, however, increases the importance of proper preform placement in the die.
While boron nitride can effectively be used in SPF titanium operations using metal dies because the dies have near-zero porosity to eliminate damaging oxidation of the boron nitride above 1500.degree. F. (816.degree. C.), it is not as effective with ceramic dies. Ceramic dies, have inherent porosity that allows sufficient air to oxidize the boron nitride to boria (i.e., B.sub.2 O.sub.3) glass which will bond the titanium worksheet to the ceramic die surface, ruining both the workpiece and the dies. Therefore, with hot forming and SPF of metals in ceramic dies, boron nitride is an ineffective release agent whose use must be avoided.